Fusarium wilt of watermelon, caused by Fusarium oxysporum f. sp.
niveum races 0, 1, 2, or 3, is found throughout the eastern United States.
Most seedless (triploid) watermelon cultivars are susceptible to all races. In
previous studies in Maryland, winter cover crops of hairy vetch (Vicia
villosa) reduced Fusarium wilt. The objectives of this study were to compare
Cahaba White hybrid common vetch (V. sativa × V. cordata) to
hairy vetch for suppression of Fusarium wilt in South Carolina and Maryland.
Cover crops of the two vetches and rye (the control) were seeded in fall 2006
and 2007 in fields naturally infested with a mixture of races 1 and 2 in South
Carolina and Maryland. In the spring after incorporating the cover crop biomass,
seedless watermelons cv. Sugar Heart (susceptible) or Revolution (moderately
resistant to race 1) were transplanted into subplots within each cover crop
whole plot. Cahaba White vetch was as effective as hairy vetch in reducing
incidence of Fusarium wilt. Both vetch varieties reduced wilt incidence by
approximately the same degree with the susceptible as with the moderately
resistant cultivar. In general, the moderately resistant cultivar yielded more
fruit (by weight) than the susceptible cultivar.

Introduction

Fusarium wilt is an important soilborne disease of watermelon (Citrullus
lanatus var. lanatus) in the mid-Atlantic and southeastern United
States (16). Frequently cropping fields to watermelon selects for the
specialized form of Fusarium oxysporum, F. oxysporum f. sp.
niveum (FON), that attacks watermelon (8). Fusarium wilt kills individual
vines or whole plants, directly reducing the number of fruit produced. In a
survey conducted in 2000 in Maryland and Delaware, FON was widely distributed
throughout production fields (17). Approximately half of the surveyed fields had
levels of the pathogen high enough to cause 50% wilt incidence on a highly
susceptible cultivar, such as Sugar Baby.

Race 2, a highly aggressive race of FON, is an emerging problem throughout
watermelon production areas in the eastern United States. Prior to 2000, this
race was limited to Texas, Oklahoma, and Florida (12). It has since been found
in five additional states: Maryland (17), Delaware (17), Indiana (5), Georgia
(3), and South Carolina (9). Although race 1 is the most prevalent race in the
mid-Atlantic region, 24% of the fields surveyed in Maryland and Delaware were
infested with race 2 (17). In South Carolina, race 2 was found in all fields
sampled (9). Recently, isolates belonging to the new race 3 were identified in
Maryland (22). Because neither diploid seeded nor triploid seedless cultivars
have resistance to races 2 or 3, watermelon crop losses due to Fusarium wilt are
increasing.

Management of Fusarium wilt was achieved in the past through a combination of
long crop rotations [up to 12 years in some cases (13)], and host resistance in
hybrid diploid, seeded cultivars (2,14). However, the effectiveness of host
resistance is dependent on which races of the pathogen are present. A few
triploid seedless cultivars have a moderate level of resistance to race 1 (11,21). However, most of these cultivars are not grown commercially for various
reasons. For example, because Seedless Sangria can be mistaken by produce
buyers and consumers for the seeded cultivar Sangria, Seedless Sangria is no
longer available.

A new management tactic for Fusarium wilt is the production of watermelon
following a fall-planted hairy vetch (Vicia villosa) cover crop that is
killed and turned into the soil in the spring (18). Incorporating hairy vetch in
the spring prior to transplanting watermelon reduced incidence of Fusarium wilt
and increased yields in Maryland by 45%, which was as good as control with
methyl bromide or Telone C-35 fumigation. Watermelon yields after a hairy vetch
cover crop increased from 35% to 65% compared to the control treatment, again,
as good as the fumigants (18). In addition, hairy vetch was effective against
both race 1 and 2 of FON. One problem with this method of control is that hairy
vetch and watermelon are hosts of Southern root-knot nematode (Meloidogyne
incognita), the predominant root-knot species in the South. A hybrid
cultivar of common vetch, Cahaba White (V. sativa × V. cordata),
that is resistant to Southern root knot nematode may provide similar benefits as
hairy vetch without the risk of increasing populations of M. incognita
(7,15).

The objectives of this study were to: (i) compare Cahaba White hybrid common vetch to hairy vetch for suppression of Fusarium wilt; (ii) compare vetch cover crop performance in South Carolina and Maryland; (iii) determine if vetch cover cropping improved control of Fusarium wilt with a cultivar moderately resistant to race 1 planted in soil infested with a mixture of races; and (iv) calculate the economic costs and returns of vetch cover cropping in seedless watermelon production.

The experiments were done in 2006 to 2007 and 2007 to 2008 in two fields of
Youngs loamy fine sand at Clemson Universitys Coastal Research and Education
Center, Charleston, SC, and in two fields of Norfolk A loamy sand
at the University of Marylands Lower Eastern Shore Research and Education
Center, Salisbury, MD. All four fields had previously been documented to contain
races 1 and 2 of FON (9,17). The experimental design was a split-plot with six
replications in South Carolina and four replications in Maryland. Whole plots
consisted of three winter cover crop treatments: hybrid common vetch Cahaba
White, hairy vetch (cultivar unspecified), and rye (Secale cereale).
Subplots consisted of two triploid watermelon cultivars with different levels of
wilt resistance, Sugar Heart and Revolution, susceptible and moderately
resistant to Fusarium wilt race 1, respectively (19). Sugar Heart was selected
as the susceptible cultivar because it is grown in both the mid-Atlantic and
southeastern regions.

In South Carolina, rye (32 lb/acre), Cahaba vetch (66 lb/acre), and hairy vetch (79
lb/acre) were seeded on 10 October 2006 and 15 October 2007. Whole plots were 100
ft long and 12 ft wide. On 7 March 2007 and 29 February 2008, cover crops were
flattened with a metal roller (Fig. 1A) and sprayed with paraquat (Gramoxone, 3
pt/acre) (Fig. 1B) (18,19). Prior to rolling, above-ground biomass was collected
from two 0.25-m² areas within each plot, dried for 24 h at 70°C, and
weighed. In both years plots were disked three times one week apart by whole
plot treatment (Fig. 1C); the disk was power washed with water between
treatments. Fertilizer (500 lb/acre 10-10-10) was applied to all plots in March
2007. In March 2008 plant-available nitrogen was calculated as 2.15% of the dry
weight of the vetch biomass [(4); Zhou and Everts, unpublished)]. No fertilizer was
applied to vetch plots, whereas rye plots received 500 lb/acre 10-10-10. Plots were
covered with black polyethylene mulch on 29 March 2007 and 26 March 2008.
Watermelons were seeded in a greenhouse on 15 March 2007 and 1 April 2008.
Seedlings were transplanted 36 inches apart within rows on 10 April 2007 and 2 May
2008. Subplots were 45 ft long and one and two rows wide in 2007 and 2008,
respectively (Fig. 1D), which was equivalent to 1210 and 2420 plants per acre,
respectively. The pollenizer SP4, highly resistant to Fusarium wilt race 1,
was inter-transplanted between every fourth seedless watermelon plant within
each row in both years. For pre-emergence weed control, ethalfuralin + clomazone
(Strategy 6 pt/acre) was applied between rows. Foliar diseases were managed in 2007
with two applications each of chlorothalonil (Bravo Weather Stik, 3 pt/acre) and
boscalid + pyraclostrobin (Pristine 38WG, 18.5 oz/acre) and one application of
myclobutanil (Rally, 2.5 oz/acre). In 2008 quinoxyfen (Quintec, 6 fl oz/acre) was
applied once. Insects were managed in 2007 with two applications of imidacloprid
(Provado, 3.5 fl oz/acre) and one application of methomyl (Lannate, 0.5 lb/acre). In
2008, bifenthrin (Sniper, 6.4 fl oz/acre), methomyl
(Lannate, 0.5 lb/acre) and esfenvalerate (Asana 9.6 fl oz/acre) were applied. Drip
fertigation with 8N-0P-8K plus minor elements was applied daily beginning 1 week
after transplanting. The N and K rates changed with growth stage: 7 lb N
and K per week from transplant to blossom, 10.5 lb N and K per week from blossom to fruit
set, 14 lb N
and K per week from fruit set to first harvest, and 10.5 lb lb N
and K per week after first harvest. The percent of plants showing symptoms of Fusarium wilt
(Fig. 1E, F) was assessed weekly in both years beginning 14 May through 25 June.
Marketable fruit was harvested, counted, and weighed on 21 June, 29 June, and 6
July 2007 and 18 June, 27 June, and 10 July 2008.

A

B

C

D

E

F

Fig. 1. (A) In the spring, cover crops were rolled.
(B) Cover crops were killed with contact herbicide.
(C) Cover crops were disked two to three times to incorporate.
(D) Single-row plots were used in South Carolina, 2007.
(E) Symptomatic plants were counted and marked weekly.
(F) Some vines were killed by Fusarium before harvest.

In Maryland, rye (140 lb/acre) and Cahaba White and hairy vetch (30 lb/acre in
2007, 45 lb/acre in 2008) were seeded on 10 October 2006 and 18 October 2007. Rates
were based on local practices. Plots were disked on 9 and 16 May 2007 and 22 and
28 May 2008. The amount of fertilizer applied was adjusted based on the amount
of nitrogen calculated from the vetch biomass. In 2007, 750 lb/acre 16-0-18 was
applied to rye plots and 456 and 575 lb/acre 16-0-18 was applied to Cahaba White
and hairy vetch plots, respectively. An additional 294 and 175 lb/acre 0-0-18 was
applied to Cahaba White and hairy vetch plots, respectively, so that all
treatments received the same amount of potassium. In 2008, 750 lb/acre of 16-0-18
was applied to rye plots and 17 and 115 lb/acre of 16-0-18 was applied to Cahaba
White and hairy vetch plots, respectively. In addition, 750 lb/acre 0-0-18 was
applied to both vetch plots. Plots were 64 ft long and one row wide and covered
with black polyethylene mulch under which a drip irrigation tube was laid.
Watermelons were seeded in the greenhouse on 2 May 2007 and 2 May 2008 and
transplanted to the field on 30 May 2007 and 3 June 2008. Seedlings were spaced
36 inches apart within rows, which was equivalent to 2420 plants per acre. The pollenizer SP4 was inter-transplanted between every other seedless watermelon
plant within each row in both years. Ethalfluralin (Curbit 3E, 2 pt/acre) and
terbacil (Sinbar WP, 3 oz/acre) were applied for pre-emergence weed control. Foliar
diseases were managed with chlorothalonil (Bravo Weather Stik, 2 pt/acre) and
boscalid + pyraclostrobin (Pristine 38WG, 15 oz/acre) applied on alternate weeks in
both years from the end of June until harvest. The percent of plants showing
symptoms of Fusarium wilt was assessed weekly from 2 to 7 weeks after
transplanting. All fruit were harvested and weighed on 13 August 2007 and 7
August 2008. Marketable sized fruit were defined as those weighing between 9 and
24 pounds apiece.

Data analysis was done with PROC MIXED (version 9.1, SAS
Institute Inc., Cary, NC). Year, location,
cover crop, and cultivar were considered fixed variables and block-by-cover crop
was used as the random variable. In preliminary analyses, there was a
significant (P < 0.05) year-by-location-by-cultivar interaction for wilt
incidence and significant year-by-cultivar and location-by-cultivar interactions
for marketable weight. Thus, data were analyzed by year and location. There were
no cover crop-by-cultivar interactions.

Effects of Cover Crops and Cultivar Resistance on Fusarium Wilt and Yield

Fusarium wilt incidence. In Maryland in both years, incidence of
Fusarium wilt symptoms on seedless watermelon plants was lower in plots seeded
to either vetch crop than in plots seeded to rye the preceding fall (Table 1).
Cahaba White vetch and hairy vetch were equally effective. In South Carolina,
Cahaba White vetch reduced incidence of Fusarium wilt in 2007, while hairy
vetch did not reduce incidence significantly. Wilt incidence was too low in
South Carolina in 2008 to detect differences among cover crops.

In all four experiments, Revolution, the seedless watermelon cultivar
moderately resistant to race 1, had a much lower wilt incidence than Sugar
Heart, the susceptible cultivar. For both locations, the difference in wilt
incidence between cultivars was greater in 2008 than in 2007.

Table 1. Final Fusarium wilt incidence (%) in plots naturally infested with
FON races 1 and 2 in South Carolina and Maryland in 2007 and 2008.

Treatment

South Carolina

Maryland

2007

2008

2007

2008

Combined

Cover
crop

Rye (control)

64
ax

2.4 a

52 a

69 a

60.9 a

Hairy vetch

52 ab

2.3 a

35 b

39 b

36.7 b

Cahaba White vetch

47 b

1.0 a

38 b

43 b

40.7 b

P
value

0.05

NS

0.01

0.01

0.0001

Cultivar

Sugar Heart (susceptible)

73 A

4.2 A

56 A

79 A

−y

Revolution (moderately
resistant to race 1)

35 B

0.4 B

29 B

21 B

−

P
value

0.0001

0.01

0.0001

0.0001

x Means within a column with the same lowercase or uppercase letters are not
significantly different based on t tests of least-squares means.

y Cultivar-by-year interaction was P = 0.004 for South Carolina and P < 0.0001 for Maryland.

Marketable yield. Cover crop treatments did not significantly affect
total weight or weight of marketable-sized fruit in any experiment (Tables 2 and 3).
Thus, the different fertilizer applications in the different cover crop
treatments also had no effects on yields. Number of marketable-sized fruit also
did not differ among cover crops (data not shown). Yields were greater in South
Carolina than in Maryland, because in South Carolina all fruit were allowed to
reach marketable size before harvest, whereas in Maryland all fruit were
harvested at one time.

In three experiments, fruit weights produced by moderately resistant cultivar
Revolution were much greater than weights of susceptible cultivar Sugar
Heart (Table 2 and 3). In South Carolina in 2007, up to half of the Sugar Heart
fruit were unmarketable because of sunburn on exposed fruit where vines had died
(data not shown). In general, the distribution of fruit among different size
categories was not influenced by cover crop or cultivar (data not shown). The
year-by-cultivar and location-by-cultivar interactions were significant because
in the 2008 experiment in South Carolina, wilt incidence was so low there was no
difference between the two cultivars (Table 2).

Table 2. Weight (tons/acre) of marketable-size and all sizes of watermelon fruit
from plots naturally infested with FON races 1 and 2 in South Carolina.

Treatment

2007

2008

Marketable

Total

Marketable

Total

Cover
crop

Rye (control)

9.5

13.6

11.2

24.1

Hairy vetch

12.1

17.1

11.3

20.4

Cahaba White vetch

11.6

16.8

13.1

21.6

Cultivar

Sugar Heart (susceptible)

7.3 Bx

13.2 B

12.3

22.7

Revolution (moderately
resistant to race 1)

14.8 A

18.4 A

11.5

21.3

x Means within a column with the same letters are not significantly different based on t tests of least-squares means, P < 0.01 in 2007 for cultivars. Cultivar-by-year interaction was P = 0.003.

Table 3. Weight (tons/acre) of marketable-size and all sizes of watermelon fruit
from plots naturally infested with FON races 1 and 2 in Maryland.

Treatment

2007

2008

Marketable

Total

Marketable

Total

Cover
crop

Rye (control)

4.4

10.6

9.6

12.7

Hairy vetch

6.7

12.4

12.7

16.1

Cahaba White vetch

6.1

11.1

12.3

15.9

Cultivar

Sugar Heart (susceptible)

1.0 Bx

6.8 B

7.5 B

10.1 B

Revolution (moderately
resistant to race 1)

10.5 A

16.0 A

15.6 A

19.7 A

x Means within a column with the same letters are not significantly different based on t tests of least-squares means P < 0.0001 in 2007 and P < 0.001 in 2008 for total yields and P ≤ 0.0001 for yield of marketable-size fruit. There were no interactions by year.

Vetch biomass. In South Carolina, aboveground biomass of vetch measured
in the spring was greater than the biomass of rye in both years, probably
because vetches were seeded at a higher rate than rye was (Table 4). Biomass of
the two vetches did not differ significantly, although the stand of hairy vetch,
16.6 plants/ft², was greater (P < 0.0001) than the stand of Cahaba
White vetch, 11.4 plants/ft², in both years at 3 weeks after emergence.
Although rye was seeded in Maryland at a higher rate than vetches were, biomass
of Cahaba White vetch was greater than the biomass of the other two cover
crops in 2007; biomass did not differ among cover crops in Maryland in 2008.
Vetches produced approximately the same amount of biomass in both locations in
2008 even though the seeding rate was 60% higher and the growth period was 2
months shorter in South Carolina than in Maryland.

x Rye (32 lb/acre), Cahaba vetch (66 lb/acre), and hairy vetch (79 lb/acre) were
seeded on 10 October 2006 and 15 October 2007. Biomass was collected prior to
rolling on 7 March 2007 and 29 February 2008.

y Rye (140 lb/acre) and Cahaba White and hairy vetch (30 lb/acre in 2007, 45 lb/acre
in 2008) were seeded on 10 October 2006 and 18 October 2007. Biomass was
collected prior to disking on 9 May 2007 and 22 May 2008.

z Means within a column with the same letters are not significantly different
based on t tests of least-squares means at P < 0.01.

Economic analysis of cover crops. Treatment-specific costs were cover
crop seed and pre-plant fertilizer. Based on actual costs for these experiments,
vetch seed was slightly more expensive than rye seed, although this cost was
partially offset by reducing the rate of nitrogen fertilizer applied to fields
cropped to vetch, because of the organic nitrogen supplied by the vetch cover
crop. We calculated that the vetches contributed on average 81 ± 29 lb/acre
of nitrogen in these experiments. Costs of the vetch treatments were higher than
the standard rye treatment in South Carolina, because vetch was seeded at a
higher rate than that used for vetch in Maryland (Table 5).

The variable production cost (which included supplies, labor, and interest on
operating capital) for drip-irrigated, seedless watermelon with polyethylene
mulch was $2696/acre (excluding pre-plant, in-bed fertilizer, which was a
treatment cost in these experiments) (1,6). Based on a total production cost of
$3125 (which included estimates for machinery, permanent irrigation fixtures,
land rent, and overhead), a grower of Sugar Heart would have lost more money
growing a rye cover crop instead of vetch in a heavily infested field in
Maryland. It is noted that gross revenue in Maryland would be higher with
multiple harvests. Net returns averaged across cultivars and locations were very
similar for the two vetches (Table 5). Averaged across the two vetches, two
locations, and two years, a vetch cover crop increased a growers net return by
$799/acre over a rye cover crop.

Table 5. Costs and returns ($) for cover crop treatments in South Carolina
and Maryland averaged across 2007 and 2008.

Cost

South Carolina

Maryland

Rye

Hairy
vetch

‘Cahaba
Whitevetch

Rye

Hairy
vetch

‘Cahaba
Whitevetch

Cover crop seed

11

155

129

48

74

74

Fertilizer

115

58

58

191

169

153

Subtotal treatment
costs

126

212

187

239

243

226

Other direct costsx

3125

3125

3125

3125

3125

3125

Gross revenue
across cultivarsy

4,140

4,680

4,940

2,800

3,880

3,680

Net return with
Revolution

1,093

2,496

2,293

1,471

2,600

2,267

Net return with
Sugar Heart

688

232

986

(2,302)

(1,173)

(1,236)

Net return across
cultivars

889

1,360

1,646

(564)

512

329

Return above rye
for Revolution

−

1,403

1,200

−

1,129

796

Return above rye
for Sugar Heart

−

(456)

298

−

1,129z

1,067z

Return above rye
across
cultivars

−

471

757

−

1,076

893

Return above rye
across
locations for Revolution

−

1,266

998

−

−

−

Return above rye
across
locations for Sugar Heart

−

336z

682z

−

−

−

Return above rye
across
locations and cultivars

−

774

825

−

−

−

x Direct costs of production include variable costs (excluding in-bed
fertilizer) of $2696 and fixed and other costs of $429 (1).

y Marketable weight of fruit at $0.20/lb.

z Represents revenue not lost.

Conclusions and Recommendations

Cahaba White vetch was as effective as hairy vetch in reducing incidence of
Fusarium wilt with a susceptible seedless watermelon cultivar and a cultivar
with resistance to race 1. As in a previous study in Maryland, there were no
interactions between cover crop and cultivar treatments (19). In the current
study, vetch cover crops reduced wilt incidence by approximately the same degree
with the susceptible as with the moderately resistant cultivar. This result
differed from previous work in Maryland, where wilt suppression was greater for
a moderately resistant than a susceptible cultivar (21). Biomass produced by
Cahaba White vetch was equivalent to or greater than biomass produced by hairy
vetch, even though hairy vetch had a higher plant stand than Cahaba White
vetch in South Carolina. Thus, Cahaba White hybrid common vetch is another
vetch, in addition to hairy vetch, that can be used by watermelon growers in the
mid-Atlantic and southeastern United States to mitigate the effects of Fusarium
wilt. Although the mechanism of the suppression is unknown, previous work
demonstrated a correlation between populations of soil bacteria and wilt
reduction (21).

The moderately resistant cultivar Revolution yielded more marketable-sized
fruit and more total fruit than the susceptible cultivar Sugar Heart in three
of four experiments with significant levels of Fusarium wilt present. Triploid
cultivars that have resistance only to race 1 of FON increase yield even when
grown in fields infested with a mixture of races 1 and 2, as reported previously
(19). The economic benefits of vetch cover crops were numerically greater with
the moderately resistant cultivar than with the susceptible cultivar. Thus,
vetch cover crops can benefit watermelon growers, regardless of the cultivar
grown. In South Carolina, one large watermelon grower planted 50 and 150 acres
of hairy vetch in 2007 and 2008, respectively [(10); Keinath, unpublished].

Acknowledgments

Funding for this project was provided by NIFA/USDA in the form of a Pest
Management Alternatives Program grant to the University of Maryland
(2006-34381-16955). Additional funding was provided by CSREES/USDA under project
numbers SC-1700161 and 2006-34287-17370 and by the South Carolina Watermelon
Association. Technical assistance was provided by Ginny DuBose, Andy Lassiter,
and Greg Baccari. Advice on statistical analysis was provided by Denis Shah.

Technical contribution no. 5815 of the Clemson University
Experiment Station.